The present invention relates to a double-heterostructure semiconductor laser, and, more particularly, to a semiconductor laser with a mesa stripe waveguide structure which continuously produces light beams at the ambient temperature in a fundamental transverse-oscillation mode.
Recently, in a technical field of optical data communications or optical information recording, double-heterostructure semiconductor lasers are increasingly receiving attention with regard to their role as a light source that generates an optical information signal. In order to fabricate a multi-layered semiconductor laser, it is necessary to strictly control the re-crystal growth of each semiconductor layer in the manufacturing process of the semiconductor laser so that the re-crystallized layer would have a uniform and high crystal quality. Semiconductor lasers have traditionally been fabricated by a liquid phase epitaxy method or LPE method; however, to more effectively satisfy this requirement, a metal organic chemical vapor deposition method (hereinafter referred to as MOCVD method as it is commonly called so among those skilled in the concerned art) has recently been developed and used instead.
The co-pending U.S. patent application Ser. No. 019,332 filed Feb. 26, 1987 (corresponding EPC Application No. 87301775.0 filed Feb. 27, 1987) proposes a double-heterostructure semiconductor laser, which is fabricated on a GaAs substrate by the MOCVD method and has cladding layers formed of InGaAlp --a new semiconductor material for a laser. In this type of laser, a p conductivity type InGaAlP cladding layer is formed to have a mesa stripe shape defined by two slanted side surfaces. The cladding layer therefore provides a linear projection or rib which defines the mesa-like waveguide channel section of a semiconductor laser. Current-blocking layers of an n conductivity type are provided to partially cover the mesa stripe-shaped cladding layer, and perform current-blocking function for the laser in a visible laser beam emission mode. It is desirable for the cladding layer that the band gap and the hole mobility within the cladding layer are as large as possible. When a mixed crystal semiconductor material is used for the cladding layer, the composition ratio is typically selected to provide a direct transitional band gap. In the disclosed semiconductor laser, the aluminum composition ratio y of the InGaAlP cladding layer is selected to be y&lt;0.35. When y&gt;0.35, the cladding layer is in an indirect transition stage so that an increase in the band gap of this layer becomes significantly gentle and the hole mobility in the cladding layer is reduced. This is not desirable for a laser which uses a pn junction to provide the current-blocking function.
According to the semiconductor laser with the above structure, it is likely to cause a lattice defect at the boundary between the cladding layer and the current-blocking layers (particularly, the boundary at the slanted portions of the mesa stripe-shaped channel). The lattice defect causes current to leak at the pn junction. When the leak current increases in the forward-bias or reverse-bias direction during emission of a laser beam, the current-blocking function of the current-blocking layers becomes insufficiently low, thus degrading the fundamental operation characteristics of the laser, i.e., causing an unstable operation in the laser beam emission mode and/or increasing noise.